Optical communication networks use wavelength division multiplexed (WDM) techniques to carry multiple traffic flows. Each lightpath uses a different wavelength channel within a defined spectral band. Conventionally, optical networks have used a fixed grid of WDM or densely wavelength division multiplexed (DWDM) optical channels for lightpaths. An optical source, optical receiver and multiplexing components along an optical path are all based on this grid of wavelength values. The International Telecommunications Union (ITU-T) has defined a grid of channels, typically with 50 GHz or 100 GHz spacing.
There are now plans to provide a more flexible grid of spectral resources, called a flexi-grid. One reason for considering a flexible grid is to accommodate the spectral needs of higher bit rate channels, such as 400 Gbit/s or 1 Tbit/s channels. The flexible grid allows a more flexible use of the limited spectral resources. A lightpath can be allocated a channel bandwidth which is suited to the transmission needs of the lightpath (e.g. a small number of spectral blocks, or a larger number of spectral blocks), and the spacing between adjacent blocks of allocated spectrum can be adjusted to match the needs of the particular transmission scheme. This can allow better use of the limited spectral resources on transmission links of the optical network.
In optical networks, it is advantageous to have the spectral resources that are available for use (free resources) located together in one or more large blocks. However, the free resources are often fragmented. That is, the free resources are located in multiple discontinuous regions of the spectrum of a link. Also, the free resources can be located in different places on different links. Fragmentation of the spectral resources can arise due to dynamic traffic conditions, unexpected network evolutions and network recovery and maintenance operations. Fragmentation can also arise due to the process of allocating a single wavelength, where possible, for an end-to-end path between a source node and a destination node. In an optical network with a flexible grid, the presence of lightpaths possibly operating at different bit rates and modulation formats and occupying a variable portion of the frequency spectrum, can further increase the problem of fragmentation of spectral resources.
The process of forming larger, contiguous, blocks of free resources is called defragmentation. Effective defragmentation techniques, also called optimisation techniques, are useful to improve the overall spectrum utilisation in flexible optical networks.
A simple example of fragmentation of spectral resources is shown in FIG. 1A. FIG. 1A shows a portion of a network comprising four nodes 10, A-D, and three links 11. There is a first lightpath 12 between nodes A and B, and a second lightpath 13 between nodes B and D. FIG. 1A also shows the spectral resources that have been allocated to the lightpaths 12, 13. The first lightpath 12 occupies k frequency slots (where k is any integer number≧1), and has f0 as the nominal central frequency. The second lightpath 13 between nodes B and D occupies k frequency slots on links B-C and C-D and has f1 as the nominal central frequency. Although there are unused frequency slots along the route A-D, they are located in different parts of the spectrum at different points along the route A-D. This scenario prevents the set up of a new lightpath from node A to node D, unless wavelength conversion is provided at node B. Wavelength conversion is undesirable as it requires additional opto-electro-opto transponders at a node.
FIG. 1B shows the same scenario after an optimisation of the spectral resources. In this case, lightpath B-D is moved from the spectral position shown in FIG. 1A (nominal centre frequency f1) to the spectral position shown in FIG. 1B (nominal centre frequency f0). Changing the spectral allocation of the lightpath 13 defragments the spectrum, enabling the set up of one or more lightpaths from A to D allocated in the released frequency slots.
A known process for defragmenting the spectrum is called Make-before-Break (MbB), which is described in Internet Engineering Task Force (IETF) document RFC 3209, “Extensions to RSVP for LSP Tunnels” at section 2.5 “Rerouting Traffic Engineered Tunnels”. Make-before-break is also described in RFC 4872 and RFC 4873. The three main steps of Make-before-Break are shown in FIG. 1C. Step 0 shows the initial situation, before implementing the Make-before-Break process, with the lightpath 13 allocated a block 14 of frequency slots with the nominal centre frequency f1. In the first step an additional lightpath between the same source node (B) and destination node (D) pair is established along the newly computed route or central frequency. A new lightpath from node B to node D is set up in a different block 15 of spectral resources with f0 as the nominal central frequency. In the second step the client traffic is switched between the two active lightpaths. In the third step the original lightpath from B to D at the nominal central frequency f1 is torn down.
There are some disadvantages associated with performing Make-before-Break operations. Make-before-Break may introduce some traffic disruption or misordering. Packet duplication or loss can occur due to traffic switching between the two lightpaths at the source node or can arise due to delay variations caused by different latencies (e.g. if different routes are considered). This can cause disruption at the service level. Make-before-Break also requires the availability of additional spare and expensive transponders at both the source node and the destination node. A further issue affecting Make-before-Break relates to the additional operations needed at the optical layer. The set up operation at step 1 of FIG. 1C and the teardown operation at step 3 of FIG. 1C varies the number of active lightpaths along the traversed links, which can have an effect on optical amplifiers traversed by those lightpaths. This can require optical power equalisation procedures, and can possibly affect the stability of other active lightpaths.
The present invention seeks to provide an alternative way to change the allocation of spectral resources to a lightpath.